This invention relates to color cathode ray tubes of the type having a shadow mask, and especially to a system for suspending a shadow mask on the faceplate of a color tube. This invention has applicability to suspension systems for shadow masks of various types, including post deflection focus masks.
Conventional color cathode ray tubes have a shadow mask assembly which includes a heavy frame to which is welded a dished, apertured mask. The frame is, by design, extremely rigid and provides the necessary rigidity for the mask. The mask-frame assembly is mounted in a conventional tube by a suspension system comprising three or four leaf springs which are welded to the frame at spaced points around the periphery thereof. These springs must be relatively stiff to suport the heavy mask-frame assembly, typically applying a load of 4-5 pounds or more to the mask-frame assembly. The springs have apertures at their distal ends which engage studs projecting inwardly from a rearward flange on the tube faceplate when the assembly is mounted in a tube. The mask-frame assembly is capable of being demounted and precisely remounted in a tube by depressing the springs to disengage the said studs. This type of system has proven to be commercially viable, however, the mask-frame assembly and the tube envelope are undesirably expensive.
The present invention is an improvement of a shadow mask suspension system constituting a radical departure from conventional and other prior art approaches to shadow masks and shadow mask suspension systems. According to that unique approach, a low cost, lightweight, non-self-rigid, torsionally flexible mask is provided. The faceplate is used to impart the necessary rigidity to the mask. A novel suspension system is provided which furnishes a mechanically rigid link between the faceplate and the mask, and yet which permits the mask to be conveniently and repeatably demounted and precisely remounted in the tube. The advantages of this system are manifold. A primary advantage resides in the appreciable savings in tube cost. Tube cost savings result from the use, in a preferred embodiment, of an envelope having a flangeless faceplate which is less expensive than the conventional flanged faceplate, and from the use of a lightweight (low mass), low cost shadow mask, preferably of one-piece, frameless construction.
A system of the type taught by this invention has imposed upon it a number of requirements and constraints not presented in conventional systems in which a rigid frame is used to impart rigidity to the mask. Before enumerating these requirements and constraints, a discussion of certain principles underlying this invention will be engaged. A shadow mask of the type with which this invention is concerned may be modeled as a rectangular four bar linkage affixed to a flexible sheet. Such a model is shown in Figure A. The four rigid bars of the linkage are designated A, B, C and D; the sheet is labeled S. As is well known, a four bar linkage is not inherently a rigid structure. The rectangular four bar linkge, in its free state, might, e.g., quite easily be skewed into a parallelogram geometry. It is evident, however, that the Figure A model cannot be skewed in its plane to take a parallelogram shape since it is affixed to the sheet S.
The linkage, can, however, be torsionally twisted about its diagonals, as shown for example in Figure B. In Figure B, the model has been twisted as follows -- the linkage bar A has been rotated toward the reader (see arrows); the linkage bar C has been rotated away from the reader. The corners 1 and 3 have been displaced upwardly and the corners 2 and 4 have been displaced downwardly. The sheet S is thus stressed convexly along diagonal 2-4 and somewhat concavely at the ends of diagonal 1-3. The model may thus be thought of as being twisted about one of its diagonals (here shown as diagonal 1-3). It can be noted that the model configuration, after twisting, is changed substantially less along its major axis M.sub.a and minor axis M.sub.i, than along the diagonals. Thus a four bar linkage affixed to a flexible sheet is relatively stiff with respect to its major and minor axes (due to the rigidity of the bars), but is relatively flexible in torsion. When torsionally flexed (twisted), about its diagonals, the corners are displaced, but points on the major and minor axes remain relatively stationary.
As will be pointed out in more detail hereinafter, the shadow mask with which this invention is concerned is similar to the described model in its mechanical characteristics.
The principles of this invention, though not limited to such application, are most useful when embodied in a color cathode ray tube having a flangeless faceplate. When such a faceplate is sealed to mating funnel after completion of the faceplate screening and mask insertion operation, the faceplate is very apt to experience a twist-wise elastic distortion due to a tolerance-related configurational mismatch between the funnel and faceplate sealing surfaces. Any such distortion will be rendered a permanent deformation when the sealing cement has cured and the sealing operation is completed. Thus, one of the necessary general requirements imposed on a mask and mask-suspension system intended for use with a flangeless faceplate is that it must be able to adapt to such twist-wise deformations of a faceplate with which it is mated. Stated another way, the mask must be capable of flexing or twisting about its diagonals in much the same way faceplates are apt to twist-wise deform in their contour during tube fabrication, and its suspension system must provide for such adaption. As will become evident as this description proceeds, the shadow mask and suspension system with which this invention is concerned are uniquely capable of meeting this requirement.
Second, and of equal significance -- with respect to any given faceplate, since the mask is non-self-rigid, the suspension system for the mask must effectively transfer the rigidity of the faceplate to the mask.
Third, the suspension system must precisely fix and hold a predetermined spatial position of the mask as a whole relative to the faceplate against translational or rotational displacement, in spite of any thermal expansion or contraction of the mask, demounting and remounting of the mask, or mechanical shocks.
Fourth, it is desirable that any thermally induced movement of any part of the mask or of any mask suspension element during tube operation be radial, rather than tangential, since radial errors can be compensated by adjusting in the beam deflection characteristic, whereas tangential errors cannot be.
Fifth, it is desirable that the system permit the mask to be conveniently and quickly demounted and remounted, preferably automatically, since in conventional factory faceplate screening practices the mask is mounted on or demounted from the faceplate many times.
A sixth general requirement is that the mask suspension system should carry a low manufacture cost. A different type of shadow mask and suspension system thereof is disclosed in the patent to Fyler -- U.S. Pat. No. 2,961,560. This patent shows a frameless shadow mask supported at a multiplicity of spaced peripheral points directly on projections from the concave screen-bearing surface of the tube faceplate. By this approach, it would appear that the rigidity of the faceplate is used to impart rigidity to the mask, thus eliminating the necessity for the mask to also be rigid. The Fyler approach would appear to suffer, however, (1) from an intolerable difficulty and inconvenience in the demounting and remounting of the shadow mask in the tube, an operation performed many times on conventional faceplate screening practices, (2) a difficulty in seating and reseating the mask uniformly on the multiplicity of support elements provided on the faceplate, (3) uncontrollability of the spatial position of the mask corners, and thereby a loss of color purity in the corners of the displayed images, (4) a probable shifting of the geometrical center of the mask upon thermal expansion and contraction thereof due to the non-equalized, frictional retention of the mask in the Fyler mask mounting system, (5) difficulty in achieving a commercially satisfactory "Q" compensation of the mask if such is necessary, and (6) a relatively high cost of system manufacture and assembly.
As will be pointed out in more detail hereinafter, this invention involves the provision of a shadow mask suspension system comprising four suspension devices, one at each corner of the tube faceplate, at least three of the devices including an axially extending cantilevered leaf spring. I have found that numerous additional specific requirements are imposed upon such a system, devolving in part from the corner location of the suspension devices, and in part from the use of a cantilevered type spring as an element of the device.
A seventh specific requirement is as follows. In order to achieve the afore-discussed fixing of the spatial position of the mask, in the context of a four-corner cantilevered spring suspension system, as described, it has been discovered that at least three of the springs must be extremely stiff in the plane of the spring. If the mask suspension springs are not sufficiently stiff in the plane thereof, i.e., in the tangential direction as mounted, and prefereably (though not necessarily) in torsion also, the mask will not always return to its bogey position (nominal assigned position) after having received a mechanical shock or after having been demounted and remounted. This fact is due largely to the mass of the mask and to friction at the points of engagement of the mask-mounted and envelope-mounted components of the mask suspension devices.
An eighth important requirement of the mask suspension system is that it provide a relatively constant and relatively low-valued radial spring loading on the mask, without the imposition of any significant moment tending to twist or deform the mask. Yet the mask must be supported against mechanical shocks which, e.g., may apply 45 G's or more to the mask. This requirement is especially important in a suspension system designed, as the present system is, especially for use with a lightweight, non-self-rigid mask capable of being distorted or deformed by an excessive loading or by a moment loading thereof.
A ninth (specific) requirement is that, in order that the suspension device not occupy a large area in the corner of the faceplate, which would require the provision of a larger-than-desired faceplate (and associated funnel), the deflection of the leaf spring to effect engagement or disengagement of the mask from the faceplate must be quite small. Further, the spring must not be so large as to require the provision of an intolerably great amount of space in the corner of the faceplate to accommodate same.
Tenth, the spring must be of a thickness, for certain embodiments of the invention, to be suitable for welding to a supporting structure. Further, the spring must not be overstressed during demounting or remounting of the mask and during thermal cycling of the tube during tube fabrication.
A prior art patent to Haas--U.S. Pat. No. 2,922,063 appears to disclose a suspension system for a shadow mask which is in some respects similar and in other respects very different from the suspension system of the present invention. Haas discloses a shadow mask having a lightweight frame to which is attached a perforate color selection mask. The mask is suspended adjacent the concave inner surface of a faceplate of the type having a rearwardly extending flange.
A suspension system is shown for suspending the mask which comprises four suspension devices located on the major and minor axes of the faceplate, each suspension device including a relatively wide leaf spring which is attached to the faceplate adjacent the seal land and extending forwardly to the mask frame. The springs are said to be "thin, flat metallic strips so that they are stiff in a lateral dimension but flexible in a direction perpendicular to the major flat face" (column 3, lines 48-51).
The Haas system is considered to have a number of major shortcomings which have perhaps been responsible for its apparent failure to have achieved commercial use. The leaf spring is apparently so flexible out of its own plane as to require that it be either screwed to a sealed-in flange (FIG. 4) or held on a faceplate-embedded stud by means of a special spring clip (55 in FIG. 5). In either arrangement, it is possible that any thermal expansion of the mask of frame would result in a moment being applied to the mask-frame assembly which would distort the mask and produce color impurity in the displayed images.
The Haas system would be further unsuited for use in the present system for failure to meet the afore-described first, fifth, and ninth requirements and perhaps others.
Yet another prior art approach is expounded by U.S. Pats. Nos. 3,450,920; 3,497,746; 3,529,199; 3,548,235; British No. 1,278,633; British No. 1,278,634; British No. 1,278,635 and British No. 1,172,334. In these systems, a shadow mask having a deep-drawn integral mask skirt, either with or without a frame, is mounted adjacent to the concave inner screen-bearing surface of a faceplate of the type having a rearwardly extending flange. Numerous ways are shown by which such a mask may be suspended on the faceplate flange by means of mask-mounted elements which are received in recesses formed integrally in the faceplate flanges. In certain embodiments the recesses are suggested for location on the faceplate flange sides. In other embodiments it is suggested that the recesses be located in the faceplate flange corners. The basic approach described in these patents would be totally incapable of meeting a number of the basic requirements imposed on a system of the type with which this invention is involved, described above. As a practical matter, it is impossible to consistently and repeatedly achieve the necessary accuracy in mask-to-faceplate registration in any mask suspension system in which large and unpredictable friction forces are produced. U.S. Pat. No. 3,529,199 also discloses a more conventional stud-spring suspension system (FIG. 2) but this too is deemed to be of little value in meeting the needs of a system of the type with which this invention is concerned.
The mask suspension systems of the referent copending applications have achieved noteworthy success in developmental tests in meeting the afore-described needs and requirements. This invention, however, represents an improvement over the systems of the said applications, as well as over the last-described prior art approach and over the Fyler, Haas, and all other known prior art systems.